1. Field of the Invention
This invention relates to an electromagnetic device, such as an electromagnetic clutch and an electromagnetic brake, in which two armatures are selectively attracted into contact with a friction surface upon energization of an electromagnet having one electric coil.
2. Prior Art
A conventional electromagnetic clutch 10 shown in FIGS. 1 and 2 comprises a rotor 11 mounted through bearing members 12 on a hollow cylindrical portion 13a of an equipment 13 in which the electromagnetic clutch 10 is used, the rotor 11 having a base portion 11a and a pair of concentrically-disposed annular recesses 11b and 11c. The base portion 11a has a friction surface 11d. The rotor 11 has a pulley portion 11e of a generally V-shaped cross-section at its outer periphery. A drive belt (not shown) connected to a drive means (not shown) such as an electric motor is received in the V-shaped pulley portion 11e for rotating the rotor 11. A yoke 15, made of a material of high permeability such as iron, is mounted on the equipment 13 and has a pair of annular hollow portions 15a and 15b disposed concentrically. First and second electric coils 16 and 17 are received in the hollow portions 15a and 15b, respectively. A pair of non-magnetic rings 18 and 19 are embedded in the base portion 11a of the rotor 11 for providing magnetic poles on the base portion 11a when the electromagnet means constituted by the yoke 15 and the coils 16 and 17 are excited. The annular hollow portions 15a and 15b are loosely received in the annular recesses 11b and 11c, respectively, so that the first and second coils 16 and 17 are disposed in opposed relation to the non-magnetic rings 18 and 19, respectively. An output shaft 19 is mounted on the equipment 13 through a bearing member 20. A hub 21 is rotatably mounted on the output shaft 19 through bearing members 22 and carries a leaf spring 23 which in turn supports a first armature 25 through a mounting member 26, the first armature 25 being disposed in closely spaced relation to an outer portion of the friction surface 11d of the rotor base portion 11a. Fixedly mounted to the output shaft 19 is a hub 27 which supports a second armature 28 through a leaf spring 30, the second armature 28 being disposed in closely spaced relation to an inner portion of the friction surface 11d of the rotor base portion 11a .
With this arrangement, when the first coil 16 is excited or energized through an associated excitation circuit (not shown), a magnetic flux is produced in the electromagnetic clutch 10 as indicated by arrows in FIG. 1 to provide a magnetic circuit, so that the first armature 25 is attracted into contact with the friction surface 11d of the base portion 11a of the rotor 11 against the bias of the leaf spring 23 and rotated together with the rotor 11. Therefore, the rotation of the rotor 11 is transmitted via the armature 25, the mounting member 26, the leaf spring 23 and the hub 21 to a first load (not shown) connected to the hub 21. Upon de-energization of the coil 16, the first armature 25 is brought out of contact with the base portion 11a of the rotor 11 under the influence of the leaf spring 23.
On the other hand, when the second coil 17 is excited with the first coil 16 de-energized, a magnetic flux is produced in the electromagnetic clutch 10 as indicated by arrows in FIG.2 to provide a magnetic circuit, so that the second armature 28 is brought into contact with the friction surface 11d of the rotor 11 under the influence of electromagnetic attractive forces and is rotated together with the rotor 11. Therefore, the rotation of the rotor 11 is transmitted via the second armature 28, the leaf spring 30 and the hub 27 to the output shaft 19 which is connected to a second load (not shown).
This conventional electromagnetic clutch 10 has been found disadvantageous, however, in that it is relatively heavy in weight and complicated in construction and hence expensive in manufacture since it employs the two coils. In addition, since the two coils are disposed in radially spaced relation, the electromagnetic clutch 10 is bulky in construction. Further, in order that the first and second loads can be driven at the same time, the magnetic fluxs must be produced by the respective coils in such a direction that the magnetic fluxs will not cancel out each other. Therefore, the associated excitation circuit is rather complicated in construction.